Electrolytic production of magnesium metal



flied my 11, 1957 March 31, 1959 gum m, 2,880,151 ELECTROLYTIC kpbvcnou0 MAGNESIUM- METAL I I 2 Sheets-Sheet 1 mmvr'oxs. L/oyaG. DeanFranc/335k OKs/ uasi: Ken Pose g,Jr. I w M {MA HTTOR/VEYS Y UnitedStates Patent ELECTROLYTIC PRODUCTION OF MAGNESIUM METAL Lloyd G. Dean,Lake Jackson, Franciszek OlstoWski,

Freeport, and Ken Posey, Jr., Lake Jackson, Tex., assignors to The DowChemical Company, Midland, Mich., a corporation of Delaware ApplicationFebruary 11, 1957, Serial No. 639,436

2 Claims. (Cl. 204-70) This invention relates to electrolytes for, andmethods of, electrolytic production of magnesium. It especially relatesto molten salt mixtures as electrolytes having a density less than thatof molten magnesium and to the method of producing magnesium therefrom.

Magnesium is currently produced on a large scale by the electrolysis ofa fused salt mixture containing magnesium chloride. These electrolyticbaths have a density greater than that of molten magnesium. Unless thecathode in the bath consists of a molten metal which alloys with moltenmagnesium produced by the electrolysis, the liberated magnesium rises tothe surface of the bath and is removed therefrom .in recovering themagnesium so produced. The use of a molten cathode with which theproduced metal becomes alloyed entails a complex subsequent separatorystep to recover the magnesium therefrom and is not economicallyfeasible. In current practice, therefore, the magnesium is permitted torise to the surface of the electrolyte. Since the gaseous products ofthe electrolysis are also lighter than the bath, and therefore rise toand evolve from the surface, either elaborate means for preventingrecombination of the magnesium with the evolving gaseous productshavebeen found necessary when producing strong chlorine or the cell mustbe swept with some sufiicient-ly unreactive gas, usually air. The :firstof these causes the cell to operate at a higher voltage, hence toconsume more power, while the latter produces a dilute chlorine andsacrifices some current efficiency.

.A further difliculty encountered with baths heavier than moltenmagnesium is that the magnesium, as it collects at the surface of thebath, is exposed relatively unprotected from .air which .gives rise to aconstant threat of burning of the magnesium, particularly at higheroperating temperatures. .As a result, .a significant amount of theliberated magnesium is reoxidized and unrecoverable.

Furthermore, the efficiency of known electrolytic methods of producingmagnesium has not been satisfactory as shown -.by the number ofkilowatt-hours of power required to produce a pound of magnesium. Thislow efficiency is due largely to an undesirably wide spacing ofelectrodes which is made necessary by the relatively high density of theelectrolyte employed, to recombination of the products of electrolysis,to oxidation, and to the fact that the character of the electrolyterequires a cell design which inherently results in uneconomical 1 Rlosses in the cell, electrodes, and external connections.

In view of the diificulties attendant upon conventional methods ofelectrolytic production of magnesium, it is a desideratum in the art toprovide an improved electrolyte for and method of producing magnesiumelectrolytically.

According to the invention we have provided an improved fused saltelectrolyte and method of producing Patented Mar. 31, 1959 earth metal,from 56 to percent of potassium chloride,-

and other chlorides of alkaline earth metals, e.g., calcium chloride andsodium chloride, to the extent that such other chlorides do not resultin an electrolyte having a density which is not at least 0.034 gram percc. less than that of molten magnesium at the temperature of the elec*trolysis. By fluoride fraction is meant that portion of afluorine-containing salt which would be elemental fluorine on analysis.In electrolyzing the improved electrolyte between a suitable anode andcathode in accordance with the invention, the magnesium is produced outof contact with the atmosphere below the surface of the electrolyte, ina highly efficient and convenient manner, and is recoverable withoutsignificant loss. The fluoride has been found to aid the collection ofthe molten magnesium therein into a coalescent mass or regulus.

A characteristic of our novel electrolyte, and method of producingmagnesium therefrom which particularly enhances its utility, is itsability to tolerate water-containing salts in the feed used to replenishthe electrolyte dur-' ing electrolysis. For example, water may bepresent in the magnesium chloride feed, employing our electrolyte, tothe extent of 30 percent and may be present in amounts up to 25 percentwithout lowering the cathode efficiency as much as 5 percent. The reasonfor the high tolerance of water is not known for certain but thepresence of potassium chloride in the electrolyte in the percentagerange encompassed by the invention appears to effect a condition in theelectrolyte which substantially inhibits the reaction of water withmagnesium chloride that the chlorine produced is not excessively dilutedby extraneous gases. This objective is attained without the sacrifice ofpower loss that is encountered by the use of baflles, partitions, orcurtains.

A third characteristic of our novel electrolyte, and method of employingit, which further enhances its utility is its low initial cost.Furthermore, although potassium chloride forms the major proportion ofthe electrolyte, his not substantially decomposed during theelectrolysis and only small losses need be made up from time to time.

The invention then consists of the improved electrolyte and method ofproducing magnesium therefrom herein fully described and particularlypointed out in the claims, reference being made to the accompanyingdrawing.

In said drawing:

Figure 1 is a plan view of an electrolytic cell with which the inventionmay be practiced.

Figure 2 is a sectional elevation along line 2-2 of Figure 1.

Figures 3 and 4 are graphs showing the operating density range andcorresponding percentages by weight of salts composing the fused saltmixtures of the invention.

Referring now to Figures 1 and 2 of the drawing in detail, there isshown steel shell 1 enclosing refractory brick setting 2. Iron pot 3,having a flanged rim, is placed in the furnace setting. The pot iselectrically insulated onthe inside with ceramic lining 4. Electrolyte5, formulated in accordance with the invention, is placed in lined pot3. The top of the cell is provided with ceramic-lined metal cover 6. AC.electrodes 7, extending downwardly through openings provided therefor incover 6, provide a means for passing an AC. through the electrolyte tosupply heat to maintain it in a molten state. The A.C. electrodes may belowered or raised by means of chain falls (not shown) according to theneed for the passage of more or less heating current. In cover 6 isopening 8 for admission of feed and access to the pot. Opening 8 isprovided with removable cover 9. Outlet 10 is provided for egress ofchlorine and other gases, if any, formed during electrolysis. Drainassem- -bly 11, having a valve therein, is provided at the lower portionof pot 3 as an alternative means for removing magnesium metal. Extendingthrough an opening in cover 6 and into pot 3 is anode 12. The degree towhich anode 12 extends is controlled by a chain fall (not shown). Steelor graphite cathode 13 is shown at the bottom of pot 3. Current leads 14and 15 are connected to the anode and cathode respectively by suitableterminals 16 and 17 respectively. Packing gland 18 is positioned aboutthe anode and packing glands 19 are positioned about A.C. electrodes 7for snug fits with and insulation from the openings provided therefor incover 6.

In carrying out the invention, the heated pot is charged withelectrolyte formulated in accordance with the invention or its separateingredients. Heat is usually initially applied to the charge byinserting a heating means such as a gas flame through opening 8 andplaying it on the charge until it is sufficiently molten to conductcurrent. The lower ends of the AC. electrodes 7 are dipped into themolten charge and current is then passed between A.C. electrodes 7 untila suitable temperature for electrolysis is reached. If desired, theelectrolyte ingredients may be melted in a separate vessel andintroduced slowly in the molten condition into the electrolytic cell.When a suitable temperature has been reached, electrolysis is effectedby applying a suitable between anode 12 and cathode 13. A suitabletemperature for electrolysis employing the novel electrolyte of theinvention is in the range of 815 degrees to 900 degrees C. andpreferably 825 degrees to 900 degrees C.

As the electrolysis proceeds, the magnesium liberated at the cathodeaccumulates at the bottom of the cell to form the molten body ofmagnesium 20. Chlorine is liberated at the anode and rises to thesurface of the electrolyte where it is withdrawn from the cell, asthrough outlet 10. The accumulated molten magnesium may be recovered bymeans of a dipper or a siphon inserted through opening 8 or by means ofthe drain assembly 11 by opening the valve therein. When the cell is inoperation, a portion of the accumulated molten magnesium 20 may beallowed to remain on the floor of the cell and thus serve as a cathode.

In the electrolysis, as the magnesium chloride in the electrolytebecomes depleted during operation of the cell containing the electrolyteof our invention, it must be replenished either at intervals orcontinuously to maintain the desired proportion of it in the fused bath.Occasional additions of potassium chloride, and a fluoride if employed,may be necessary to maintain their proper proportions in theelectrolyte.

During the electrolysis a small amount of non-metallic insoluble mattercalled sludge may accumulate at the bottom of the cell. The fluoride, ifadded as already mentioned, also aids in settling the sludge whichstratifies below the molten magnesium in the cell. Such sludge may beremoved either by a dipper as in the case of the produced metal or bydraining through drain assembly 4 11 by opening the valve therein.Desludging operations are necessary after protracted operation but nomore frequently than is now required in current practices.

It is understood that the cell of Figs. 1 and 2 is illustrative of butone form of cell for use in the practice of the invention and that otherforms of cells and modifications of that shown may be used with thenovel electrolyte according to the method herein described. Generally,however, the cell required for carrying out the invention iscomparatively simple in design since no precautions are necessary toprevent recombination of the chlorine and magnesium.

In the preferred embodiment of the invention, the electrolyte includesfrom 0.1 to 1.0 percent, but preferably from 0.25 to 0.75 percent, byweight of the fluoride fraction, i.e. the elemental F portion, of afluoride of an alkali or alkaline earth metal. In the case of CaF forexample, since F constitutes of CaF the amount of CaF to be added is of0.25 to 0.75 or between about 0.51 and 1.54 percent.

The magnesium chloride content of the electrolyte is to be at least 5weight percent because electrolytic dissociation of potassium chloridebecomes objectionable at less than 5 percent by weight of magnesiumchloride in the electrolyte. On the other hand, the percentage ofmagnesium chloride is to be no higher than that which will result in adensity differential between the electrolyte and the molten magnesium,at operating temperatures, of at least 0.034 g. per cc. which occurswhen the magnesium chloride is about 44 weight percent at 850 to 900 C.in the presence of 1 percent CaF and substantially free from othermaterials. If the density of the electrolyte is not at least 0.034 g.per cc. less than the molten magnesium, the magnesium deposition tendsto become erratic due to the tendency of some of the magnesium to remainsuspended in the electrolyte.

The cell suffers some loss of efliciency when the magnesium chloridecontent of the electrolyte is between 30 percent and 44 percent sotherefore more eflicient operation is obtained when the magnesiumchloride is not over 30 percent. The preferred range for magnesiumchloride is from 8 to 28 percent by weight of the fused electrolyte.

Referring to Figures 3 and 4 in detail, lines a and a respectively showthe density of the electrolyte of the invention having varyingpercentages of KCl and MgCland containing 1 percent of Calat 850 and 900C. respectively. Lines b and b of Figures 3 and 4 respectively show thedensity of magnesium metal at the same two temperatures respectively.Lines 0 and 0' respectively show the maximum permissible density of theelectrolyte, i.e. a density which is 0.034 g. per cc. less than that ofmagnesium at the temperature of the electrolyte. A and A, the shadedportions, represent, respectively, the density operating areas between 5percent MgCl and P and P percent MgCl; in which the density of theelectrolyte is 0.034 g. per cc. less than that of magnesium metal at thetemperatures of 850 and 900 C.

l It is manifest from Figures 3 and 4 that P and P' reprey Thetemperature of electrolysis was 850 sent the highest percentages of MgClpermitted at 850 and 900 C., respectively. It may be readily observedthat much greater tolerance of other chlorides than those of magnesiumand potassium as described hereinbefore, e.g., chlorides of sodium andcalcium, may be tolerated at percentages of MgCl approaching the 5percent minimum requirement, than those approaching P and P percent ofMgCl Tables I and II set out the operating conditions and pertinent dataobtained in a series of examples of the practice of this invention.Anhydrous feed was used in the examples of Table I and hydrous feed,containing 25 percent water, was used in the examples of Table II. C.The density difierential between the electrolytes and molten magnesiumin the examples set out in the tables and 0.054 gram per cc.

is between 0.052v

TABLE I Examples m which anhydrous MgCl feed used Anode Avg. Comp. ofElectrolyte, Wt. Cathode Anode Oath Kwh./ Length Average to OathpercentCurr. Curr. Curr. 1b. Mg Examples 01 Run, Voltode Den., Den., E11, Pro-Hours age 1 Spacing Amps./ Amps] Perduced 5 (ln.) K01 MgGh CaF, MgO in?cent 3 7. 5. 88. 2 11. 7 none 0.05 79. 4 6.9 18.5 5. 85 3 79. 3 19. 7 0.51 0.01 3.6 3. 3 74.0 7. 9 18. 1 5. 85 3 81.1 18.1 0.33 0.08 3. 2 3.090.0 6. 5 17. 6 5. 0 1. 5 79. 1 18. 6 0. 39 0. 03 2. 3 2.1 90.4 5. 518.0 5. 3 1. 5 79. 3 20.5 0. 93 2. 9 2. 7 88. 8 6. 0 43. 0 5. 2 1. 5 80.4 18. 9 0. 58 0 3. 1 2.9 75.0 6. 9 165. 5 5. 8 1. 5-3 80.0 29. 0 0.433.0 2. 8 77. 2 7. 5 22. 2 5. 4 3 79. 2 19. 2 0. 58 2. 2 2. 0 82. 2 6. 517. 4- 5.9 3 79.3 19.3 0.45 3.0 2. 8 91. 3 6. 5 16.2 4. 9 3 80.2 18.20.61 2. 7 2. 5 90. 6 5. 4 15. 3 4. 6 3 80. 6 17.9 0. 46 2. 6 2. 4 91. 05. 0 14. 0 4. 9 3 82. 5 16.1 0. 38 2. 8 2. 5 92. 2 5. 3 19. 0 4.9 3 80.817. 5 0. 67 2. 7 2. 5 80.0 6. 1 17.8 4. 8 3 86. 7 11. 6 0.70 2. 4 2. 273. 2 6. 6 18.3 5. 3 3 89. 6 8. 4 1.00 0. 03 2. 6 2. 4 86.0 6. 2 19. 286. 8 3 79. 0 20. 0 1. 00 1. 6 5. 3 83. 7 8. 1 430. 00 6. 2 1. 5 79. 020. 0 1. 00 9. 3 91.1 6. 8

TABLE 11 Examples m which MgCl hydrate 6 used Elec- Avg. Comp. ofElectrolyte, Cathode Anode Cathode Length Avg. trode Weight percentCurr. Curr. Curr. Kwh./ Examples of Run, Volts 1 Spacing, Den., Den.,E11, 1b. Mg 5 Hours Inches AmpsJ Amps] percent 3 K01 MgCla Cali: MgO miin.

1 Voltage applied between anode and cathode. 2 Calculated on totalimmersed area. (1 d wt. of Mg Pro uce X100 percent Cathode currentefificlency wt. of Mg Possible according to Faradays Law.

4 Not determined.

5 Kwh./lb. Mg was calculated according to the formula: 6 Average percentwater in feed= percent.

TYPICAL ANALYSIS OF GAS EVOLVED FOR WHICH ANALYSIS WAS MADE IN EXAM-PLES OF TABLE II Ch H01 0 0: 03 N; C O

Referring to the tables above, it can be seen that high efliciency andlow power consumption in an electrolytic cell employing the novelelectrolyte are obtainable by the invention. Examples 4, 5, 9, 10, 11,12 and 17 show cathode efficiencies of at least 90 percent and powerconsumption as low as 5.0 and not over 6.8 kw. h. per pound of magnesiumproduced. The examples shown in the tables were run at anode currentdensities varying between 2.0 and 9.3 amperes per square inch ofimmersed area and at anode-to-cathode spacings of 1.5" and 3". Theresults show that the eflect of such variations in current density andspacing, as well as the presence of magnesium oxide and water to theextent shown, is not appreciably detrimental. Although no coalescentagent is essential, as shown by Example 1, much better coalescence ofthe magnesium occurred in the examples in which CaF was present as acoalescent agent; the collection of the magnesium into a body and itsremoval from the cell were facilitated by the presence of the CaF Thefluoride was conveniently added as fluorite or fluorspar. Example 1 alsofurther shows that magnesium oxide may be tolerated whether or not afluoride is also present.

No serious difliculties were encountered in operating appliedvoltageXamp.Xhours 100011). of Mg produced Mg Pmducedof the cell at 900C. This fact is an unexpected discovery. Heretofore, temperatures above800 C. have been thought detrimental to the economical production ofmagnesium metal by electrolysis of molten salts containing magnesiumchloride. This erroneous thought associated with known electrolytes waslikely based on opinions concerning metal fogging in the region of thecathode, high vapor pressures of the salt components and moltenmagnesium, and increased burning of magnesium metal at the surface ofthe electrolyte. These detrimental effects are not encountered in thepresent method.

Impurities up to 1 or 2 percent by weight normally present in the feed,such as magnesium oxide, and traces of salts and oxides of other metalsamong which are those of iron, copper, nickel, silicon, manganese, lead,titanium, boron, aluminum, and chromium, may be tolerated. Since most ofthese metals deposit at a lower potential than does magensium, thespecifications of the magnesium to be produced will predetermine thepermissible amounts of such metals.

Among the advantages of the invention are: production of relatively puremagnesium metal below the surface of the electrolyte; production ofchlorine in a concentrated form as a by-product; comparatively simplecell requirements; high-temperature operation Without adverse efiects;negligible loss of metallic magnesium; high elficiency as reflected bylow power consumption per pound of magnesium produced; wide tolerance ofWater in the feed; low initial cost of the salt in which electrolysistakes place; employment of an alkali or alkaline earth fluoride, thepresence of which readily coalesces the molten magnesium and facilitatesits separation from the electrolyte and sludge.

Having thus described our invention, What we claim and desire to protectby Letters Patent is:

1. In the method of producing magnesium metal and chlorine gas of highpurity by electrolyzing a molten salt bath comprising KCl and MgCl whichis of less density than the molten magnesium at between 815 and 900 C.the improvement which consists of adding to the bath during electrolysishydrous MgCl feed containing between 0.5 and 4.0 moles of H 0 per moleof MgCl to maintain the MgCl content of the bath between 5 and 30percent by weight and to maintain the density of the bath at not lessthan 0.034 gram per cubic centimeter less than that of molten magnesiumat the temperature of electrolysis.

2. In the improved method according to claim 1, the step of adding MgFto the bath in suflicient amount to 8. provide up to 1 percent by weightof the fluoride fraction thereof based on the weight of the bath.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Ser. No. 340,402 (A.P.C.), published May 18, 1943.

1. IN THE METHOD OF PRODUCING MAGNESIUM METAL AND CHLORINE GAS OF HIGHPURITY BY ELECTROLYZING A MOLTEN SALT BATH COMPRISING KCI AND MGCL2WHICH IS OF LESS DENSITY THEN THE MOLTEN MAGNESIUM AT BETWEEN 815* AND900*C. THE IMPROVEMENT WHICH CONSISTS OF ADDING TO THE BATH DURINGELECTROLYSIS HYDROUS MGCL2 FEED CONTAINING BETWEEN 0.5 AND 4.0 MOLES OFH2O PER MOLE OF MGCL2 TO OBTAIN THE MGCL2 CONTENT OF THE BATH BETWEEN 5AND 30 PERCENT BY WEIGHT AND TO MAINTAIN THE DENSITY OF THE BATH AT NOTLESS THAN 0.034 GRAM PER CUBIC CENTIMER LESS THAN THAT OF MOLTENMAGNESIUM AT THE TEMPERATURE OF ELECTROLYSIS.